This invention relates to a multi-range load cell scale which comprises a light-weight measuring load cell and a heavy-weight measuring load cell, and which displays the output of the light-weight measuring load cell when the weight of the measured object is not greater than a predetermined value and displays the output of the heavy-weight measuring load cell when the weight is greater than a predetermined value.
In this specification, the multi-range load cell scale means a scale with a plurality of load cells for measuring weight in different scale ranges which are continuously coupled with one another.
An example of a multi-range load cell scale has two load cells. The weighing range of the first load cell is 0 to 2.5 kg, and the unit weight or one scale division is 1 g in this range. In measuring weight of 2.5 kg or less, the output from the first load cell for light weight measurement is displayed for measured weight indication. The weighing range of the second load cell is 2.5 kg to 10 kg, and the unit weight is 5 g in this range. In measuring weight of greater than 2.5 kg but not more than 10 kg the output of the second load cell for heavy weight measurement is displayed. In this type of a multi-range load cell scale, due to the different weighing characteristics of both load cells, there is a problem with non-continuous displays occurring in the vicinity of the borderline between the two range (hereinafter called switching point). For example, if the load weight is increased successively, the weight reading displayed may increase suddenly at the switching point, or a reading lower than the preceding reading may be displayed. This problem will be discussed in detail, referring to FIG. 1. First of all, the required conditions will be given. One scale division of the light-weight measuring or light-range load cell is 1 g. If the weight displayed is 2500 g, the true value of the load weight is in the range of 2500.+-.0.5 g. One scale division of the heavy-weight measuring or heavy-range load cell is 5 g. If the weight displayed is 2500 g, the true value of the load weight is in the range of 2500.+-.2.5 g.
In FIG. 1, when the true weight value and the displayed weight value form a continuous line, as indicated by straight line A, an ideal display condition is obtained and no problem of discontinuity occurs at the switching point. When an instrumental error, or an inherent error of each load cell causes readings of equal amounts above or below the true value, the weighing characteristic has a variation along straight line B or C. Therefore, also in this case, there is no problem with discontinuity.
An extreme example of displayed weight change at the switching point will be explained. Suppose the light-weight measuring load cell of the scale has characteristic as shown by straight line B, and the heavy-weight measuring load cell has characteristic as shown by straight line E. In this case, when the measured weight of the light-weight measuring load cell exceeds the switching point of 2500 g, the weighing characteristic of this load cell shifts from straight line B, along straight line F, to straight line E. At this point where straight line F meets straight line E, the measured weight is 2503 g. Since this value falls within the 2505.+-.2.5 g range, the displayed weight is 2505 g. The weighing range in which the output of this load cell scale will be displayed as 2500 g is exceedingly narrow. However, this is the unavoidable lot of the multi-range load cell scale.
Here is another example. Suppose the light-weight measuring load cell has the characteristic shown by straight line C, and the heavy-weight measuring load cell has the characteristic by straight line D. In this case, when the measured weight value exceeds 2500 g, the weighing characteristic is shown by straight line G, and shift to straight line D. To be more specific, at the point where straight line G meets straight line D, the measured weight is 2497 g. This value falls within the range of 2495.+-.2 g. Therefore, the displayed weight is 2495 g. If a load weight is then increased by 0.5 g, the displayed weight will be 2500 g according to the output of the heavy weight load cell. Thus, the displayed weight will be 2499 g, 2500 g, 2495 g, 2500 g, and 2505 g according as the load weight increases. The sequence of the displayed weight increases is irregular. The decrease in the displayed weight at the switching point is very serious problem for scales.